Turbine wheel



Feb. 18, 1964 R, H. AsPlNwALL 3,121,555

TURBINE WHEEL.

Filed Deo. 19, 1960 ATTORNEY United States Patent O 3,121,555 TURBiNE WHEEL Robert H. Aspinwall, Indianapolis, Ind., assigner to General Motors Corporation, Detroit, Mich., a corporation oi Delaware Filed Dec. 19, 1960, Ser. No. 76,824 1 Claim. (Cl. 253-39) This invention relates to a rotor construction, and more particularly to a turbine rotor wheel construction.

The normal temperature gradient existing in solid type gas turbine engine turbine wheels or spacers is a major contributor to cracking or buckling of the wheel rims. The hot rim and relatively cool hub imposes high compressive stresses in the rim and high tensile stresses in the hub. This is an acute problem especially in small turbine wheels seven inches or under in diameter, for example. Since they experience the same thermal difference, the gradient is much steeper and more severe.

This invention relates to a wheel construction that not only prevents iiuid leakage across the web, which can not be tolerated in many installations, but controls the thermal and centrifugal stresses in the web and rim to prevent rim cracking. This is accomplished by providing a wheel having a hub and rim joined by a circumferentially iluted or corrugated web that eliminates the tangential thermal stress in the web, minimizes the tangential thermal stress in the rim and increases the centrifugal tangential stress in the rim to counteract the remaining rim tangential thermal stress.

Therefore, it is an object of this invention to provide a rotor having a iluted continuous web to prevent leakage across the web and to minimize wheel rim compressive stresses by rendering the wheel web incapable of carrying a load in the circumferential direction.

Other objects, features and advantages will become apparent by reference to the succeeding detailed description of the invention and to the drawings illustrating the preferred embodiment thereof; wherein,

FGURE l is a side elevational View of a rotor wheel embodying the invention; and,

FGURES 2 and 3 are cross-sectional views taken on planes indicated by and viewed in the directions of the arrows 2 2 and 3-3, respectively, of FiG. 1.

in most gas turbine engines, the turbine wheel or spacers are subject to extremely high temperature gradients from the hub to the rim. The temperature may be, say, 506 F., at the hub, while the rim, which is subject to the heat of the exhaust gas products, may be at a temperature of 1500 F. or more. In a normal solid disk or wheel construction having a continuous rim, the rim will expand thermally at a much higher rate than the flat web. This results in restraining tangential web stresses sucient at times to crack the rim. Such temperature gradients therefore induce high tangential compressive stresses in the rim which must be reduced if rim cracking is to be avoided. This invention does this. High speed rotation of the wheel also induces tangential centrifugal stresses on the rim but acting in an opposite direction and not of the magnitude of the thermal stresses. However, these are made use of by the invention and increased to balance the remaining thermal tangential compressive rim stresses in a manner to be described to substantially eliminate this problem.

A rotating wheel subject to a severe temperature gradient between the hub and rim is therefore subject essentially to two sets of stresses, thermal and centrifugal. if the web is constructed so that it is incapable of carrying a load in a circumferential direction, the tangential restraint oiered by the web to the expansion of the rim would be essentially removed and the web would expand radially freely without restraint. The high thermal tangential compressive stresses normally induced in the rim would then be minimized. The circumferentially corrugated or lluted turbine wheel of this invention provides such a web. However, with the severe temperature gradient existing between the continuous rim and the web portion immediately beneath it, the rim is so much hotter than the web and its expansion rate is so much greater, that the web still offers some restraint to the expansion of the rim. These remaining thermal compressive stresses are balanced, however, by the oppositely ac"- ing centrifugal stresses which have been increased as a result of this iluted web construction as will now be described.

As stated previously, the centrifugal stresses acting on the rim of a rotating dat solid wheel induces tangential centrifugal stresses acting in a direction opposite to that of the thermal tangential stresses. From Hookes Law and Poissons ratio effect, the deflection or radial expansion of a solid rotating wheel due to centrifugal eiiects varies essentially as a function of the radial minus the tangential (multiplied by Poissons ratio) centrifugal stresses. Therefore, if the tangential centrifugal web stresses are reduced to Zero or minimized so as to be almost negligible, the resultant wheel deflection or expansion will be increased for the same value of the radial centrifugal stress. Increasing the wheel expansion move-s the rim out radially since it is incapable of supporting itself as a free ring. The increase in radius, therefore, induces a considerably higher tangential centrifugal stress in the continuous rim. This increased tangential centrifugal stress is then sutiicient to offset and substantially balance the remaining thermal tangential compressive stresses in the rim to reduce the objectionable rim stresses to an acceptable level. Thermal growth of the wheel is then simply the expansion radially from the hub.

By way of example, in a normal solid flat rotating wheel of selected diameter subject to a severe tem-'Jerature gradient, thermal compressive stresses in the rim may be, say, 60,000 p.s.i., whereas the oppositely acting tangential centrifugal forces may be only 26,000 psi. However, with the construction or" this invention, the iiuted web reduces the tangential compressive stresses in the rim to, say, ZSGO-ZQOGO psi., and the centrifugal rim tangential stresses are increased by the radial expansion of the wheel to substantially 25,GO0-30,000 p.s.i. to balance these thermal stresses.

To minimize the tangential compressive rim stresses, therefore, the web must be made to stretch circumferentially upon thermal growth of tie rim, i.e., the expansion occurs through die web without restraint.

The construction shown in the figures accomplishes this objective. FiG. l shows a solid or one-piece turbine wheel l@ having a circular hub i2 and an annular continuous rim 14. A number of circumferentially `spaced turbine blades lo are integrally cast on the outer periphery or" the rim and are adapted to extend into the path of the combustion products of a gas turbine engine, for example, to be driven thereby.

Integrally connecting the hub and rim is the circu 1.- ferentially continuous annular web portion i3 of the wheel having a radial taper decreasing outwardly from the hub. rlhe thin portion 2li of the web immediately below the rim and extending approximately to the web radial median point is circumferentially corrugated or fiuted to provide a number of narrow connected web portions 22. The integral web portions are bent approximately at right angles to each other and connected at their adjacent edges to provide an accordion lii'e shape to the web in cross section as best seen in FIG. 3. The web thus forms an integral assembly with the hub and rim through which no fluid medium will leak. With this construction, the ability of the web to support a tangential load is therefore removed since the web will stretch circumferentially under thermal and centritugal tangential loads. Tangential expansion of the rim is therefore permitted with minimized tangential stresses. The angles that the web portions 22 make with each other are such that will permit the material to withstand the circumferential expansion of the web and yet produce no more than negligible bending stresses.

While the wheel assembly has been described as a cast wheel or spacer for use in a gas turbine engine, it will be clear that many modifications to the construction could be made without departing Yfrom the scope of the invention, and that it would have uses in many installations other than that described, as long as the temperature gradients and other conditions exist.

In operation, therefore, and assuming the wheel is rotating at high speed in an environment producing the extreme temperature gradient described, circumferential growth ot the wheel rim under the thermal and centrifugal loads occurs with the web portion 2&1 circumferentially expanding to minimize rim tangential compressive stresses. The resulting increase in the tangential centrifugal stresses offsets the remaining thermal compressive stresses to minimize the stresses to an acceptable level. The continuous- 1y uted or waved construction therefore provides a solid leakproof wheel with little circumferential strength.

From the foregoing, therefore, it Vwill be seen that the invention provides a rotatable wheel construction capable of withstanding high thermal and centrifugal loads without cracking or buckling of the rirn or wheel.

I claim:

A rotor stage for a turbine comprising, inrcombination, a wheel having a rim and blades on the rim of the wheel, the blades and rim being exposed to very hot motive fluid in Operation of the turbine and the blades normally driving the wheel at high speed during operation of the turi bine, the wheel further comprising a hub at the center thereof and an annular substantially imperforate web extending from the hub to the rim and rigidly connected to the hub and rim, the hub normally being much cooler than the rim during operation of the turbine with a resulting steep temperature gradient from the hub to the periphery of the wheel of the order of several hundred degrees Fahrenheit per inch, with the result that greater thermal expansion of the rim than the web in operation produces compressive stresses in the rim due to force exerted on the rim by the web and opposed by the centrifugal hoop stresses developed in the rim due to highspeed rotation of the rim and blades, characterized by a structure of the web corrugated between the hub and the rirn with the ridges of the corrugations extending in the radial direction, and with the depth of the corrugations increasing in the direction from the hub to the rim, so

that the resistance of the web adjacent the rim to circun ferential expansion is substantially reduced and thereby the said temperature-gradient-induced compressive stresses in the rim are substantially reduced to a value more nearly balanced by the said centrifugal hoop stresses in the rim in operation.

Rete-rentes Cited in the le of this patent UNTED STATES PATENTS Germany Aug. 16, 1879 

